Synthetic resin films having a volume resistivity within a semiconductive range are used in various fields as raw materials of charge controlling members. Typical technical fields, in which the charge controlling members are used, include image forming apparatus such as copying machines, laser beam printers and facsimiles of an electrophotographic system (including an electrostatic recording system).
In, for example, a copying machine of the electrophotographic system, an image is formed by a series of steps including (1) a charging step for uniformly and evenly charging the surface of a photosensitive member, (2) an exposure step for exposing the charged surface of the photosensitive member in a patterned form, thereby forming an electrostatic latent image, (3) a developing step for applying a developer (toner) to the electrostatic latent image on the surface of the photosensitive member to form a visible image (toner image), (4) a transfer step for transferring the toner image on the surface of the photosensitive member to a transfer material (for example, transfer paper or an OHP sheet), (5) a fixing step for fusion-bonding the toner image on the transfer material, (6) a cleaning step for cleaning a remaining toner on the surface of the photosensitive member, and (7) a charge-eliminating step for extinguishing a remaining electric charge on the surface of the photosensitive body.
In such an image forming apparatus, a great number of members respectively having various forms such as belts, rollers, drums and blades are arranged for bearing functions in the above-described respective steps. Examples of such members include a charging member (for example, a charging belt or charging roller), a photosensitive drum (for example, a photosensitive layer and a belt-like or roller-like support for supporting the layer), a developing member (for example, a developing roller or developing belt), a developer layer thickness-controlling member (for example, a toner layer thickness-controlling blade), a transfer member (for example, a transfer belt, intermediate transfer belt or transfer roller), a cleaning member (for example, a cleaning blade), a charge-eliminating member (for example, a charge-eliminating blade, charge-eliminating belt or charge-eliminating roller) and a paper-conveying member. Belt members generally have a form of an endless belt or tube. Roller members are covered rollers with a coating layer of a resin or rubber coated on a roller base.
Since the respective steps described above require to precisely control static electricity or electric charge, many of the members used in the respective steps are required to have moderate conductivity. In a charging system making use of, for example, a charging belt, voltage is applied to the charging belt, and the charging belt is then brought into contact with the surface of a photosensitive member, thereby directly apply an electric charge to the surface of the photosensitive member so as to charge it.
In a developing system making use of a non-magnetic one-component developer, a developing roller is arranged in apposition to a photosensitive member, a toner is applied to the surface of the photosensitive member in a charged state by frictional force between the developing roller and a toner supply roller, this toner is uniformly leveled by a toner layer thickness-controlling blade, and the toner is then transferred to an electrostatic latent image on the surface of the photosensitive member by electric attraction force.
In a transferring system making use of an endless belt, a transfer material is conveyed by the endless belt, an electric charge having a polarity opposite to that of the toner is applied to the endless belt to form an electric field for transfer, whereby the toner image is transferred to the transfer material by Coulomb's force.
Many of such various members as described above are required to have moderate conductivity, more specifically, a volume resistivity within a range of 102 to 1014 Ωcm, preferably 103 to 1014 Ωcm at the whole of such members or at least surface layers thereof for developing the respective functions. Since these members are semiconductive, whereby their charge controlling functions can be developed, they may be said to be charge controlling members. In recent years, members formed by a synthetic resin material to which moderate conductivity has been imparted have come to be commonly used as such charge controlling members.
Even in a printer of an ink-jet system, respective stems such as adsorption, conveyance and separation of paper, and cleaning of attached substances are practiced by controlling an electric charge of a paper-conveying member (for example, a belt) or the like, and various kinds of charge controlling members are used therein. Wall paper, sheathing materials for OA apparatus, and the like made of a synthetic resin are also required to have semiconductivity from the viewpoint of anti-dust collection.
A charge controlling member formed by a synthetic resin material is preferably narrow in a scatter of volume resistivity with its location in addition to the fact that it has a volume resistivity within the semiconductive range. The volume resistivity thereof is more preferably substantially even. For example, if a charging belt wide in the scatter of volume resistivity is used, it is impossible to evenly charge the surface of the photosensitive body. If a transfer belt wide in the scatter of volume resistivity is used, it is impossible to exactly transfer the toner image on the surface of the photosensitive body to a transfer material. As a result, it is impossible to obtain an high-quality image.
The charge controlling members formed by the synthetic resin material are required to have high durability. When the charge controlling member is an endless belt, it is driven over a long period of time by at least two rolls. When the member is a roller member, it is rotated at a high speed. Therefore, such charge controlling members require to have sufficiently high durability to withstand such severe driving conditions.
As mechanical properties, both modulus in tension and tensile elongation at break are desirably excellent in particular. For example, if the modulus in tension of a belt is too low, the belt is distorted, so that the durability of the belt itself is impaired, and moreover in the case of an intermediate transfer belt, it forms the cause of distortion and color drift of a toner image transferred on the belt. If the tensile elongation at break of a charge controlling member is too low, its flexibility becomes insufficient, so that its breakage by entrainment of foreign matter, or the like is easy to occur.
Since the charge controlling members are often used in a high-temperature atmosphere, and high voltage of 100 V to several kV or higher may be applied to charge controlling members and the like installed in an electrophotographic copying machine in some cases, they are exposed to the risk of firing by sparks or heating. Therefore, the charge controlling members formed by the synthetic resin material are required to have excellent heat resistance and flame retardancy.
A film formed from a resin composition obtained by dispersing conductive carbon black in a fluorocarbon resin exhibits a volume resistivity within a semiconductive range and is excellent in heat resistance and flame retardancy. However, this film is wide in a scatter of volume resistivity with location and hard to be subjected to incineration disposal after use.
A film formed from a resin composition obtained by dispersing conductive carbon black in a thermoplastic polyester resin involves a problem that creep at 70° C. is high, in addition to the fact that it is wide in a scatter of volume resistivity with location. A belt too high in the creep tends to deform attending on the form of a driving roller and cause image failure by the distortion of the belt. When the charge controlling members or apparatus such as image forming apparatus equipped with the charge controlling members are transported by truck or ship, the temperature in the interior of the truck or ship often becomes high as about 70° C. Therefore, the charge controlling members are required to have a low creep at a high temperature of about 70° C.
The interior of an image forming apparatus such as a copying machine of an electrophotographic system is in a relatively high-temperature state during its operation. The charge controlling members formed by the synthetic resin material require to have heat resistance to the extent that they are neither deformed nor fusion-bonded to any other member under such high-temperature conditions. In particular, tension is applied to belt members such as a transfer belt by at least two driving rolls even during suspension, so that such members require to be small in permanent deformation such as elongation at a high temperature, in other words, extremely low in creep at a high temperature. The heat-resistant temperature required of the charge controlling members varies according to their uses and the design of an apparatus equipped with the charge controlling members. However, it is about 50 to 70° C. in the case of a transfer belt.
A polyimide resin is excellent in various properties such as heat resistance, and films and belts formed from a resin composition obtained by dispersing conductive carbon black in the polyimide resin are known. However, many of polyimide resins are difficult to be subjected to melt extrusion, so that it is necessary to form a film, belt or the like by a wet forming method making use of a varnish containing a polyimide precursor, and so vast production cost is required. A scatter of volume resistivity of a formed product with location is also wide by uneven dispersion of the conductive carbon black in the varnish.
Poly(ether ether ketone) (PEEK) has properties as a non-crystalline polymer in combination by moderately reducing its degree of crystallinity according to the design of a molecular structure though it is a crystalline polymer. In other words, PEEK has excellent chemical resistance, fatigue resistance, toughness, abrasion resistance, slidableness and heat resistance. PEEK is also excellent in creep characteristics at 70° C. Further, PEEK has a high modulus at a high temperature and is excellent in impact resistance and flexing resistance. Further, PEEK exhibits high flame retardancy and scarcely generates smoke and toxic gasses upon its combustion.
Therefore, various kinds of semiconductive molded products making use of PEEK have heretofore been proposed. For example, there have been proposed a base material of a photosensitive member for electrophotography that is obtained by injection-molding a molding material with conductive powder contained in PEEK in the form of a pipe (Japanese Patent Application Laid-Open No. 3-48858), a molded product obtained by press-molding a conductive resin composition with conductive carbon black, graphite and polytetrafluoroethylene to PEEK (Japanese Patent Application Laid-Open No. 3-91556) and a wafer carrier obtained by injection-molding a resin composition with conductive carbon black incorporated into PEEK (Japanese Patent Application Laid-Open No. 9-36216).
However, it has been extremely difficult to produce a semiconductive film, which has an even thickness and is narrow in a scatter of volume resistivity with location and excellent in mechanical strength, using a resin composition with a conductive filler added to PEEK.
For example, a biaxially stretched film can be produced by biaxially stretching a non-crystalline PEEK sheet (Japanese Patent Application Laid-Open No. 4-101827). When the resin composition with the conductive filler added to PEEK is used to form a non-crystalline sheet, and the non-crystalline sheet is biaxially stretched, however, the sheet is broken upon the stretching, so that it is extremely difficult to produce a satisfactory biaxially stretched film. When the resin composition containing PEEK and the conductive filler is also used to produce a uniaxially stretched or biaxially stretched film, the resultant film tends to become wide in a scatter of volume resistivity with location.
It is known to obtain a non-crystalline or crystalline PEEK film by drawdown. When a resin composition with a conductive filler dispersed in PEEK is used to produce a semiconductive film by the drawdown method, however, it is difficult to control the thickness of the resulting semiconductive film evenly, and moreover the semiconductive film tends to be easily tore in its flowing direction. Further, according to this method, the volume resistivity of the resulting semiconductive film is liable to be uneven due to molecular orientation of PEEK.
For example, there has been proposed a process for producing a tubular film by using a resin composition with 2 kinds of conductive carbon black, which are different in DBP oil absorption, added to PEEK under conditions of a spiral die temperature of 385° C. and an internal cooling temperature of 20° C. (Japanese Patent Application Laid-Open Nos. 7-85722 and 7-90110). However, it is difficult to obtain a semiconductive film having an even thickness under such film-forming conditions. The resultant semiconductive film also tends to be tore in its flowing direction.
There has been proposed a process in which a resin composition with conductive carbon black added to PEEK is formed in the form of a tube by extrusion, and the extrudate is immediately cooled (Japanese Patent Application Laid-Open No. 2000-275974). However, it is difficult to control so as to give an even thickness even according to this process, and the resultant semiconductive film is wide in a scatter in volume resistivity with location and tends to be tore in its flowing direction.